1. Field of the Invention
The invention relates generally to a device for measuring fluid flow, and more specifically, to electronically measuring flow through a fluid pipe by mass flow rate and converting the data to a volumetric flow rate. The invention is particularly useful for measuring sewage flow in sewage systems.
2. Background of the Invention
Most residential utilities can be directly metered thus allowing the provider to charge a fee based on the resident's usage of those utilities. Direct measurement of residential sewage discharge has been difficult to accomplish. Direct metering of fluid flow in a pipe is typically done in a conduit that is completely full of a homogeneous fluid of relatively constant density.
Mechanical means are often used within the pipe when measuring fluids that are not very viscous. The flow of viscous or hazardous fluids is often measured by introducing an acoustic signal through the fluid. These methods cannot be used with sewage because it is most often conveyed by gravity through a pipe that is partially full. Further, sewage is a mixture of liquid and solid material of different compositions and densities making acoustic measurement difficult and inaccurate. Systems exist for measuring larger flows of sewage that have become more liquefied such as those found in main collection pipes, but are prohibitively expensive for use on individual building pipes.
Municipal sewage flow is typically represented by volume. Many sewage utilities charge their customers according to the volume of fresh water used as measured by the volume of potable water flowing into a building through a volumetric flow meter. This indirect measurement can prove difficult to justify, especially when customers use large quantities for activities such as irrigation, car washing, and filling swimming pools. These types of activities utilize large quantities of fresh water which will not flow into the municipal sewer system. Further, this method cannot be used when a public potable water supply is unavailable.
Because of the difficulty in correlating water usage to sewage discharge, many utilities charge a flat rate for sewer service. This practice is often deemed unacceptable by residents as well. Large families usually contribute a greater volume of sewage to the municipal system than individuals who live alone; however, in a flat rate system they pay the same amount as a large family.
Direct measurement of sewage flow rates in municipal collection systems is typically conducted by the system owner for the purposes of analyzing system performance. The techniques used to accomplish these efforts are generally far too expensive to utilize for the purposes of daily metering to determine individual usage. The few devices that have been developed to measure sewage flow by volume have not gained widespread acceptance. These devices collect sewage in a hopper which tips when a predicted volume of sewage is obtained. Each time the hopper tips, one unit is counted. The potential for the solid materials to adhere to the side or bottom of the hopper and providing ineffective operation or false readings are a likely cause of concern for collection system operators.
A more practical means of measuring sewage flow by mass rather than volume is needed. By assuming the sewage flow to be mostly water, the mass flow rate can be converted to a volumetric flow rate.